1. The Field of the Invention
The present invention relates to the formation of low contact resistance VLSI contacts, vias, and plugs. More specifically, the present invention is directed to a new structure for a low contact resistance contact, via, or plug having a diffusion barrier, as well as a method for creating such a structure.
2. The Relevant Technology
Recent advances in computer technology and in electronics in general have been brought about at least in part as a result of the progress that has been achieved by the integrated circuit industry in electronic circuit integration and miniaturization. This progress has resulted in increasingly compact and efficient semiconductor devices, attended by an increase in the complexity and number of such devices aggregated on a single integrated circuit wafer. The smaller and more complex devices, including resistors, capacitors, diodes, and transistors, have been achieved, in part, by reducing device sizes and spacing and by reducing the junction depth of active regions formed on a silicon substrate of an integrated circuit wafer. The smaller and more complex devices have also been achieved by stacking the devices at various levels on the wafer.
Among the feature sizes which are being reduced in size are the contact structures through which electrical contact is made between discrete semiconductor devices bn the varying levels of the wafer. These contact structures include contacts, vias, plugs, and other structures whereby electrical connection is made to discrete components of semiconductor devices located at the varying levels of integrated circuit wafers. In order to continue in the process of reducing integrated circuit size, however, new contact structure formation methods are required which overcome certain problems existing in the art.
For instance, contact structures have historically been formed from aluminum or aluminum alloy metallization. Aluminum, however, presents the problem of spiking. Spiking results in the dissolution of silicon from active regions of the semiconductor devices into the aluminum metallization and the dissolution of aluminum into the active regions. Spiking generally occurs as a result of the tendency of aluminum, when it contacts the silicon substrate directly at temperatures of about 450.degree. C. or more, to eutectically alloy with the silicon substrate. When such a reaction occurs, silicon is dissolved into the aluminum, and there is a tendency for silicon thus dissolved to be precipitated at a boundary between the metallization layer and the active region as an epitaxial phase. This increases the resistivity across the contact structure. Furthermore, aluminum is diffused into the active region from the metallization layer and forms an alloy spike structure which can cause unwanted short circuit conduction between the active region and the underlying silicon substrate.
Contact openings have more recently been metallized with tungsten with the formation of what is known as a "tungsten plug." The tungsten plug formation process does not incur spiking, but has proven problematic for other reasons, however, and these problems are heightened by the continuous miniaturization of the integrated circuit and the modern "stacked" construction of such circuits.
The tungsten plug is typically deposited by CVD in an atmosphere of fluorine, which attacks silicon, creating "worm holes" into the active region. Worm holes can be formed from this reaction extending completely through the active region, thereby shorting it out and causing the device to fail. As a further problem associated with the tungsten plug structure, the tungsten metallization complicates the contact formation process because it does not adhere well directly to silicon or oxide.
In order to eliminate the problems associated with the reaction between the silicon substrate and the metallization material, prior art methods have typically employed a diffusion barrier structure which is provided between the metallization material and the active region and which blocks the reaction between the active region and the metallization material. The diffusion barrier prevents the interdiffusion of silicon and aluminum. It also provides a surface to which the tungsten will adhere and prevents fluorine from diffusing into the active region.
Prior art FIGS. 1 through 4 of the accompanying drawings depict one conventional method known in the art of forming contact structures having a diffusion barrier. As shown in FIG. 1, a contact opening 18 is first etched through an insulating layer 16 overlying an active region 14 on a silicon substrate 12. Insulating layer 16 typically comprises a passivation material of intentionally formed silicon dioxide in the form of borophosphosilicate glass (BPSG). Contact opening 18 provides a route for electrical communications between active region 14 and the surface of insulating layer 16. As shown in FIG. 2, a titanium layer 22 is sputtered over contact opening 18 in a further step, and coats the exposed surface of active region 14.
A high temperature anneal step is then conducted in an atmosphere of predominantly nitrogen gas (N.sub.2). Titanium layer 22 reacts with active region 14 during the anneal and is transformed into a dual layer. In forming the new dual layer, the lower portion of titanium layer 22 overlying active region 14 reacts with a portion of the silicon in active region 14 to form a titanium silicide (TiSi.sub.x) region 26 seen in FIG. 3. Concurrently, the upper portion of titanium layer 22 reacts with the nitrogen gas of the atmosphere to form a titanium nitride (TiN.sub.x) layer 24 also seen in FIG. 3. Titanium silicide layer 26 provides a conductive interface at the surface of active region 14. Titanium nitride layer 24 formed above titanium silicide layer 26 acts as a diffusion barrier to the interdiffusion of tungsten and silicon, or aluminum and silicon, as mentioned above.
The next step, shown in FIG. 4, is deposition of the metallization layer. In Tungsten plug formation, metallization is achieved by the chemical vapor deposition of tungsten to form metallization layer 20. Titanium nitride layer 24 helps improve the adhesion between the walls of the opening and the tungsten metallization material. It also acts as a barrier against the diffusion of metallization layer 20 into the active region 14, and vice-versa.
It should be apparent from the above discussion that tungsten plug formation is an involved and time consuming process. Accordingly, one drawback of the tungsten plug structure, like most other contact structures of the prior art, is the many steps required for forming it. The high number of steps is due to, among other things, the need to form a diffusion barrier in the contact opening and the difficulty of doing so while maintaining consistent sidewall coverage.
A further problem involved with the tungsten plug structure is the poor step coverage provided by current tungsten plug formation methods. FIG. 5 depicts the results of a typical attempt to deposit tungsten over titanium nitride layer 24. Cupping, or "bread loafing", of tungsten metallization layer 20 on the surface of contact opening 18, seen in FIG. 5, is a typical problem in the depicted prior art process flow step. A result of the cusping is that the contact is closed off, and cannot be completely filled. Incomplete filling results in a void area, also known as a "keyhole," that is formed within tungsten metallization layer 20. This keyhole is detrimental because it can open up during further processing steps, where material which could corrode or corrupt the tungsten layer can make its way into the keyhole. Also, the void in the center of the conducting metallization layer in the contact causes an increase in contact resistance.
As a further problem associated with the tungsten plug structure, titanium nitride layer 24, which is necessary as a diffusion barrier, has relatively high resistivity. The higher resistivity raises the contact resistance of the contact structure, which in turn has a tendency to lower the speed of the semiconductor devices being formed.
Thus, it is apparent that a contact structure and a corresponding method for forming the contact structure are needed which overcome the problems existing in the prior art. Specifically, a contact structure is needed which has a low resistivity for creating a contact structure with low contact resistance, which structure can form a sufficient diffusion barrier, and which adheres well to oxide sidewalls such that sidewall coverage of an intermediate material is not needed. A method of forming the contact structure is also needed which can be conducted with fewer steps than the methods of the prior art, and which provides better step coverage of the metallization layer in the contact opening.